Many rear-wheel drive and four-wheel drive cars as well as trucks have CV (constant velocity) joints. CV joints or homokinetic joints allow the drive shaft to transmit power though a variable angle, at constant rotational speed, preferably without an appreciable increase in friction or play. In front-wheel drive cars, CV joints deliver the torque to the front wheels during turns.
There are two types of CV joints that are most commonly used: a ball-type and a tripod-type. In front-wheel drive cars, ball-type CV joints are used on the outer side of the drive shafts (outer CV joints), while tripod-type CV joints are mostly used on the inner side (inner CV joints). The motions of components within CV joints are complex with a combination of rolling, sliding and spinning. When the joints are under torque, the components are loaded together which can not only cause wear on the contact surfaces of the components, but also rolling contact fatigue and significant frictional forces between the surfaces.
Constant velocity joints also have sealing boots of elastomeric material which are usually of a bellows shape, one end being connected to the outer part of the CV joint and the other end to the interconnecting or output shaft of the CV joint. The boot retains the grease in the CV joint, and keeps out dirt and water.
Not only must the grease reduce wear and friction and prevent the premature initiation of rolling contact fatigue in a CV joint, it must also be compatible with the elastomeric material of which the boot is made. Otherwise there is a degradation of the boot material which causes premature failure of the boot, allowing the escape of the grease and ultimately failure of the CV joint. It is one of the most common problems with the CV joints that the protective boot cracks or gets damaged. Once this happens, in addition to the escape of the grease, moisture and dirt get in, causing the CV joint to wear faster and eventually fail due to lack of lubrication and corrosion. Usually, outer CV-joint boots break first, as they have to endure more movement than the inner ones. If a CV joint itself is worn out, it cannot be repaired; it will have to be replaced with a new or reconditioned part. The two main types of material used for CV joint boots are polychloroprene rubber (CR) and thermoplastic elastomer (TPE), especially ether-ester block co-polymer thermoplastic elastomer (TPC-ET).
Typical CV joint greases have base oils which are blends of naphthenic (saturated rings) and paraffinic (straight and branched saturated chains) mineral oils. Synthetic oils may also be added. It is known that said base oils have a large influence on the deterioration (swelling or shrinking) of both boots made of CR and TPC-ET. Both mineral and synthetic base oils extract the plasticizers and other oil soluble protective agents from the boot materials. Paraffinic mineral oils and poly-α-olefin (PAO) synthetic base oils diffuse very little into especially boots made of rubber material causing shrinkage, but on the other hand naphthenic mineral oils and synthetic esters diffuse into boot materials and act as plasticizers and can cause swelling. The exchange of plasticizer or plasticizer compositions for the naphthenic mineral oil can significantly reduce the boot performance, especially at low temperatures, and may cause the boot to fail by cold cracking, ultimately resulting in failure of the CV joint. If significant swelling or softening occurs, the maximum high speed capability of the boot is reduced due to the poor stability at speed and/or excessive radial expansion.
In order to solve the aforesaid problems, U.S. Pat. No. 6,656,890 B1 suggests a special base oil combination comprising 10 to 35% by weight of one or more poly-α-olefins, 3 to 15% by weight of one or more synthetic organic esters, 20 to 30% by weight of one or more naphthenic oils, the remainder of the combination being one or more paraffinic oils, and, further, a lithium soap thickener, and a sulphur-free friction modifier, that may be a organo-molybdenum complex, and at least one molybdenum dithiophosphate (MoDTP), and a zinc dialkyldithiophosphate and further additives such as anti-oxidants, extreme pressure additives, and tackiness agents. However, the friction coefficient and the wear of grease compositions according to U.S. Pat. No. 6,656,890 B1 as measured in SRV (abbreviation for the German words Schwingungen, Reibung, Verschleiβ) tests needs to be improved. This holds in particular for the friction coefficient at an early stage of the running-in process, e.g., measured at about 6 minutes.